1. Field of the Invention
The present invention relates to an optical semiconductor device and a method for manufacturing the same and, in particular, to a deep ultraviolet light emitting device technology using a group III-V compound semiconductor.
2. Background Art
Nitride semiconductor materials containing nitrogen as a group V element are in the spotlight in the field of semiconductor light emitting devices such as light emitting diodes and laser diodes that use a pn junction, and are being researched and developed. Nitride semiconductors such as AlN, GaN, and InN are direct transition semiconductors and ternary and quaternary mixed crystals have the characteristic of being able to emit light in the range from infrared to deep ultraviolet by setting compositions as appropriate to change band-gaps.
Attention is being paid to semiconductor light emitting devices that use an AlGaInN quaternary mixed crystal as a material of a light emitting layer to emit light in an ultraviolet range (see JP Patent Publication (Kokai) No. 9-64477A (1997), for example). It is reported that a light emitting peak wavelength of an AlGaInN layer can be set in a wavelength region less than or equal to 360 nm and its internal quantum efficiency can be improved to a level comparable to an InGaN layer, although the AlGaInN layer contains indium.
However, it is difficult or, if possible, expensive, to produce a high-quality and large-area substrate for epitaxial growth made of a nitride semiconductor. Therefore, a sapphire substrate, for example, needs to be used as a substrate for epitaxial growth in manufacturing a semiconductor light emitting device that uses a nitride semiconductor. FIG. 17 shows the relationship between the lattice constant and band-gap energy (corresponding to wavelength) of an InAlGaAs-based quaternary mixed crystal. Also shown is the wavelength of an ultraviolet gas laser. As shown in FIG. 17, in order to produce a solid-state light emitting device in a short-wavelength region, the Al composition of AlGaN needs to be increased to increase the band-gap energy. Accordingly, the difference in lattice constant from the sapphire substrate (0.473 nm) increases. The increase in the mismatch between lattice constants poses a problem that the threading dislocation density in a nitride semiconductor film increases. Threading dislocations reduce the internal quantum efficiency of the semiconductor light emitting device. Therefore, it is necessary to address the problem of the dislocation to improve the internal quantum efficiency of semiconductor light emitting devices.
An object of the present invention is to increase the emission intensity of deep ultraviolet light by using an AlGaInN-based material, in particular, an AlGaN-based material, as the material of a light emitting layer.